Keyboards are essential in user interface equipment of various types, including calculators, typewriters, computer terminals, and the like. An electronic keyboard in its most elemental form is simply an array of switches which may be operated by the user to convert physical motion to electrical signals. The switches may take various forms. Make-or-break mechanical contact switches become degraded by mechanical wear and contamination and, additionally, have a tendency toward "contact bounce." Attempts to avoid these problems have led to the development of capacitive switches. These involve a capacitance having one plate which is movable by depression of the key. The resulting change in the capacitance value is sensed to verify the key depression. A common way of sensing the capacitance value is to charge the capacitor by a current source to a reference voltage. The required time for the charging can be used to determine whether the capacitance value is high or low by comparing it to some reference value. For example, the reference value may be a reference capacitor charged by a similar current. The number of individual leads to the keys of a capacitive keyboard can be reduced by the use of key scanning techniques. These involve locating key closures by sensing the capacitance values of entire rows and columns of keys to identify closed keys at their common intersections. Examples of some capacitive keyboards and scanning techniques for them are described in the following:
U.S. Pat. No. 4,211,915, issued July 8, 1980 to D. E. Miller et al. PA0 U.S. Pat. No. 4,305,135, issued Dec. 8, 1981 to J. P. Dahl et al. PA0 U.S. Pat. No. 4,405,917, issued Sept. 20, 1983 to T. Y. Chai PA0 U.S. Pat. No. 4,414,538, issued Nov. 8, 1983 to Schnizlein PA0 "Matrix Scanning Logic for a Capacitive Switching Keyboard," by John W. Volpe, in Computer Design (USA) vol. 12, No. 1 84-88, Jan. 1973 PA0 "Capacitive Keyboard FET Sense Circuit," by E. G. Crabtree et al., in IBM Technical Disclosure Bulletin, Vol. 24, No. 5, Oct. 1981
One type of capacitive keyboard which is of particular interest commercially is the film, or membrane keypad, which is a laminated structure of two membranes. Each carries a printed circuit pattern separated by an insulating membrane containing a perforation associated with each key area. The circuit patterns of the membranes include mutually opposed capacitance plates registered with the perforations. The pressing of one of the circuit pattern membranes into a perforation results in an increased capacitance for the opposed plates.
A troubling aspect of the manufacture of membrane keypads is that variations in the thickness dimensions of the perforated insulating membrane or in the thickness of other insulating layers which might be present between the opposed capacitance plates result in significant changes in the values of the capacitances from one keypad to the next. As a result, it is necessary to adjust separately for each keypad the reference value to be used for verifying key closures. This adds considerably to the manufacturing cost of the product.